What does this mean? (This is from a will where half of the money will go to a special needs trust...?

The rule against perpetuities is one of the most complicated parts of estate planning to explain. Basically, common law disfavors and prevents property from being held perpetually in trust, and therefore, voids any agreement (varies from state to state) which does not end twenty-one years after a life in being, or one generation from lives presently in being plus twenty-one years. Example, Son George gets the property and upon his death, his son gets the property and so on. the property must be transferred within 21 yrs after said persons death.